The mode of operation of a device for emitting a laser beam will be recalled in brief. It principally comprises an amplifier medium and pumping sources, which inject energy into the amplifier medium. This amplifier medium, which is in the form of a rod, may be a crystal or alternatively a doped glass. The laser beam to be amplified subsequently passes through the amplifier rod one or more times by means of optical devices, for example comprising mirrors; during each pass, it extracts a part of the energy injected during the pumping and is thus amplified in the amplifier rod. For an amplifier rod of cylindrical shape, the energy deposited during the pumping is generally confined in that part of the amplifier which is delimited by the diameter of the pump beam.
In this type of configuration of a laser beam amplification device, a parasitic phenomenon referred to as transverse lasing occurs between the time at which the energy is deposited in the amplifier rod by optical pumping and the time at which it is extracted by the beam to be amplified.
This phenomenon is associated with the creation of a laser sub-cavity in the amplifier rod along an axis transverse to the longitudinal axis of the amplifier rod, the changes in the refractive index at the amplifier rod-environment interface fulfilling the function of mirrors for this sub-cavity. The transverse lasing takes place when the oscillation condition of this sub-cavity is satisfied, that is to say when there is conservation of energy over the return path inside the sub-cavity, or in other words when the transverse gain G compensates for the losses P of the sub-cavity.
In what follows, a crystal will be taken as an example of an amplifier rod; it may of course be replaced by doped glass.
FIG. 1c represents the transverse optical gain G in a cylindrical amplifier crystal 1 (FIG. 1a) of length e, pumped through both its faces S1, S2 by a pumping laser beam 4 of diameter L. If the linear gain density is denoted by g0, the small-signal gain gps is equal to g0×e in the longitudinal direction Ox and to g0×L in a transverse direction perpendicular to Ox. Usually, L≧e.
Since the optical gain G is proportional to egps, it follows that:eg0·L>>eg0·e 
The optical gain G in the transverse direction is therefore much greater than the optical gain G in the longitudinal direction, that is to say in the direction of the laser beam to be amplified.
The transverse lasing is manifested by rapid removal of the energy stored in the crystal, caused by uncontrolled transverse stimulated emissions, at the cost of the laser beam which is intended to be amplified.
This transverse lasing is particularly problematic in the case of solid amplifier media with high gains and large dimensions (typically a gain g0 of 0.88 and a pump diameter of 70 mm). For example, it prevents the generation of femtosecond laser pulses with a very high power, typically of the order of one petawatt, using a Ti:sapphire crystal pumped with high energies of the order of 100 J.
Until now, there have been two main types of solutions for suppressing this transverse lasing: those which consist in reducing the gain for the parasitic beam and those which consist in increasing the losses for the parasitic beam.
The first are little used and not well appreciated, because the problem is that reducing the gain for the parasitic beam also involves reducing the gain for the main beam. One elegant solution has been proposed by the Applicant, which consists in dividing up the available pumping energy and sending it to the pumped material at different times. This solution has formed the subject of French Patent Application No. 0413734 “Dispositif èlectronique de suppression du lasage transverse dans les amplificateurs laser haute ènergie” [Electronic device for the suppression of transverse lasing in high-energy laser amplifiers].
Most of the solutions, however, are based on increasing the level of losses for the parasitic oscillations.
A first possibility is to replace the air around the surface Σ connecting the faces S1 and S2 of the crystal 1 with water, the advantage of which is that it reduces the reflection coefficient at the interface (the refractive index changes from 1 to 1.33, while the material generally has a refractive index of between 1.5 and 1.8) and improves the cooling of the crystal in a region where a large amount of heat is deposited. This solution has formed the subject of French Patent Application No. 04411815. However, this solution is not entirely satisfactory because the reflection coefficient at the interface is still too high, and the 800 nm radiation can furthermore be reflected by the mechanical mounting and sent back to the material.
The solutions currently used in fact consist in replacing water with a liquid whose refraction index is identical or extremely close to that of the material (the term index matching is then used) and in adding a material which absorbs the 800 nm radiation to this refraction index matching liquid 21: this absorber material is also in the liquid state (it is generally a dye) and is mixed with the index matching liquid. Thus, the photons amplified at 800 nm perpendicularly to the axis are not reflected at the interface with the material by virtue of the refraction index matching liquid (and therefore they cannot pass through the gain zone a second time and be amplified even further), and then they are absorbed by the dye. This technique, described in Patent Application FR 2 901 067, works well at eliminating the transverse oscillations for laser systems based on titanium-doped sapphire because, owing to the low repetition rate of the pumping lasers (at most 0.1 Hz), the thermal load in the titanium-doped sapphire crystals has been limited (at most 100 J of pumping at 0.1 Hz producing a thermal load of from 6 to 7 W, taking the amplification efficiency into account) and architectures with an refraction index matching liquid+dye mixture not circulating around the titanium-sapphire crystal have been perfectly able to meet requirements.
However, the technology of pumping lasers has been developed substantially over the last few years, and it has now become possible to provide one hundred joules with a repetition rate of between 1 and 5 Hz; very close to 10 Hz may be achievable in the future, which will give an average pumping power of the order of one kilowatt and thermal deposition of the order of 600 to 700 watts in the crystal.
At this level, it is no longer effective to employ radial removal of the heat (represented by the arrow 10) using the refraction index matching liquid+dye mixture, the heat capacity of which is much less than that of water. The thermal properties of the mixture used (refraction index matching liquid for providing the index matching and dye absorbing the parasitic laser emission) do not permit satisfactory removal of heat. This is because the index matching liquid is a poor thermal conductor, which induces, when increasing the repetition rate of the lasers, a parabolic temperature profile illustrated in FIG. 1b, degrading the Strehl ratio (which is a beam quality coefficient) and inducing a short thermal focal length and wavefront aberrations.
One solution consists in replacing the index matching liquid with a cold finger in metallic contact with the surface Σ of the crystal. This makes it possible to reduce the thermal effects, but not satisfactorily when the average power exceeds 400 W. Furthermore, such a cryogenic device is heavy, expensive and subject to vibrations, and does not make it possible to suppress the transverse lasing.
There are currently also devices for emitting a laser beam which comprise an amplifier medium in the form of a solid plate with pumping and thermal extraction in the longitudinal axis, and which comprise a different solid material on the side (the surface Σ), which fulfils the functions of index matching and an absorber; the material is either “welded” to the amplifier medium or placed in contact by molecular adhesion. These borders or rings are not always technologically achievable, however, depending on the materials.
Consequently, there still remains a need for a device for emitting a laser beam which simultaneously satisfies all the aforementioned requirements, particularly in terms of suppressing the transverse lasing, cooling and simplicity of use.